Members of the Myc family of nuclear protooncogenes (c-, N- and L-Myc) play central roles in the control of normal growth and development and in genetic pathways linked to cellular transformation and apoptotic cell death. Accumulating structural, biochemical and genetic evidence affords the view that the function of Myc family oncoproteins in these diverse processes relates in part to their roles as sequence-specific transcription factors. Myc family proteins possess a multifunctional amino-terminal domain with transactivation potential, a region rich in basic amino acid residues responsible for sequence-specific DNA-binding activity, and a carboxy-terminal alpha-helical domain required for dimerization with another bHLH/LZ protein, Max. All known biological and biochemical activities of Myc are highly dependent upon its association with Max. In addition to its key role as an obligate partner in transactivation-competent Myc/Max complexes, Max can also repress Myc-responsive genes through the formation of transcriptional-repression complexes with members of the Mad family.
Several lines of evidence support the view that Mad and Mxi1 are important tumor suppressors. First, the addition of Mad or Mxi1 can dramatically reduce the oncogenic activity of Myc/Ras in the REF cooperation assay. Second, Nisen and coworkers (Chen, J., Willingham, T., Margraf, L. R., Schreiber-Agus, N., DePinho, R. A., and Nisen, P. D., Nature Medicine 1:638-643 (1995)) have shown that adenoviral constructs encoding Mad profoundly inhibit the proliferation and tumorigenicity of established human tumor cell lines. Third, Mad and Mxi1 map to cancer hotspots that are altered in a broad spectrum of different tumor types. Lastly, the preliminary assessment of Mxi1 knockout mice indicates that Mxi1-deficiency is associated with a cancer-prone condition.
From a mechanistic standpoint, the ability of Mad and Mxi1 to act as potent anti-Myc agents is dependent upon a short amino-terminal alpha-helical domain that allows for association with a mammalian protein that is structurally homologous to the yeast transcriptional repressor SIN3. The mechanistic basis for the mouse Sin3-mediated repression appears to be mediated in part through the recruitment of: (1) NcoR, a nuclear co-repressor that directly impacts on Pol II activity and (2) HD-1, a histone deacetylase that deacetylates histone H3 and H4 tails resulting in a condensed and less accessible nucleosomal arrangement. Structure-function studies of Sin3 indicate that its interaction with HD-1 is a critical requirement for Sin3-mediated anti-oncogenic activity in the context of Myc-induced cellular transformation.
Two domains known to be absolutely essential for Myc oncogenic activity have been mapped to the C-terminal bHLH/LZ structure and an amino-terminal segment designated Myc homology region II (MHRII). The inventors have recently cloned the drosophila homologue of Myc and have demonstrated that both of these signature features are conserved and, more strikingly, that drosophila Myc gene is oncogenic in mammalian cells. Over-expression of an MHRII fragment has been shown by others to inhibit the ability of Myc or E1a to cooperate with activated Ras in the REF assay. This finding, coupled with the formation of a specific complex between MHRII and a nuclear factor, suggests that MHRII oncogenic activity is dependent upon its ability to interact with an unidentified nuclear factor.
Myc has been shown to be directly involved in the genesis and progression of many different tumor types. Moreover, even when Myc deregulation is not the principal genetic lesion in human cancers, many cancer-associated lesions affect signaling pathways that feed into Myc and require Myc function in order to maintain the malignant phenotype.
Accordingly, there is a great need for the discovery of and characterization of proteins that interact with the MHRII of Myc. The isolation and characterization of proteins that interact with the MHRII of Myc allows for the design of agents that interact directly with MHRII to affect the regulation of Myc and ultimately target the actions of Myc. Targeting the actions of Myc oncoprotein can potentially have a significant impact on a very wide variety of human cancers, as well as many disorders arising from deregulated cellular growth or survival such as auto-immune disorders and psoriasis, among others.